This invention pertains to the field of electronic thermometers, and is particularly applicable to the field of clinical electronic thermometers employing a probe for measurement of a patient's temperature, although it is equally applicable to other temperature measurement fields.
Electronic thermometers in the health care field generally take the form of a hand-held probe having an elongate shank portion with a thermistor or other temperature sensitive element therein, and a disposable probe cover or sheath fitting over the tip and shank of the probe, which is disposed and renewed for each use for anti-contamination reasons. The handle of the probe typically connects through a cable to a small chassis which includes the measurement and display circuitry, although with increasing miniaturization of electronic components, such functions can conceivably be contained in the handle of the probe. The chassis, if separate from the handle, is generally adapted for mounting on a cart or clipboard and is preferably battery operated so that a nurse or other medical personnel can take the thermometer, along with a supply of probe covers, when making rounds of patients.
Electronic thermometers offer a great number of advantages over conventional glass and mercury thermometers for use in the health care field. Among the advantages of electronic thermometers are the elimination of sterilization procedures for glass thermometers, made possible by the use of disposable covers; elimination of the possibility of broken glass if a thermometer is dropped; a digital temperature display to eliminate temperature reading errors; and with proper circuit design and calibration, higher accuracy and resolution is possible with accurate measurement and display of tenths of a degree Fahrenheit being easily attainable.
However, despite the above advantages of electronic thermometers, they have thus far met with only moderate acceptance in the health field. One might suppose that the reason for this is the higher cost of the unit compared with glass thermometers, but this is not believed to be the reason. The relatively low and often decreasing cost of electronic components, plus the time and labor savings potential by elimination of sterilization and handling problems with glass thermometers makes electronic thermometers economically attractive. Instead, the principal problem that has held back acceptance of electronic thermometers is their slow time response from the time the thermometer is inserted in the patient until a final stabilized reading is obtained. The problem with time response is not basically an electronic problem, since the electronic circuitry is capable of responding at rates far in excess of that of the mechanical systems involved. The problem is simply that a thermometer probe, whether an electronic thermometer probe or a glass thermometer, represents a certain amount of mass and heat capacity, and when inserted from room temperature into a body cavity it cannot change temperature instantaneously, but instead approaches its final temperature more or less exponentially. It often takes around three minutes before a final stabilized temperature is measured, either with a prior art electronic thermometer or with a glass thermometer. With the measurement time at about three minutes an electronic thermometer has an inherent disadvantage because of its cost, and under those circumstances it is often more convenient and economical to administer conventional glass thermometers to a number of patients at once for later reading, instead of having a nurse wait for a stabilized reading on an electronic thermometer at each patient.
The problem of slow time response has long been recognized in the field of electronic thermometers, and a number of different techniques have been proposed to speed up readings. One type of approach taken in the prior art is to design the probe tip and probe cover for maximum heat transfer between body tissues in contact with the cover and the actual thermistor or other sensing element. These approaches have included using a disposable probe cover that has a metal tip for higher heat conductance or one that uses an extremely thin and close fitting plastic tip. While these techniques do improve response somewhat, they cannot by themselves eliminate the unacceptable time lags, for reasons that will be pointed out in detail below.
Other prior art electronic thermometers have used extrapolation or estimation techniques to predict the final temperature value, based upon the rate of temperature rise during an initial short period following insertion of the probe into the patient. Such extrapolation or estimation techniques are based upon certain assumptions, including the assumption that the probe and cover always start each measurement from a predictable room temperature, and that the measured temperature rise of the sensor is due to a single time constant function. Unfortunately, these techniques compromise the accuracy of the system, since the estimated result depends upon the recent thermal history of the probe and cover, the ambient temperature at the time of measurement if different from that at the time of calibration, and the rate at which the probe is introduced into the patient. These variables can upset the assumptions upon which the extrapolation is based, leading to erroneous results. Accordingly, with extrapolation type prior art systems, it is still necessary to wait the full measurement time for accurate readings in important cases.